Weighing scales such as, for example, bathroom scales, have long been known wherein a weighing platform is resiliently mounted with respect to a casing and wherein a load transmission system, pivotally mounted within the casing, serves to transmit the load acting on the platform to a load cell located within the casing so as to generate an analog signal corresponding to the load.
Typically, such known weighing scales are provided with a split casing comprising displaceable upper and lower sections resiliently mounted with respect to each other. The upper section constitutes a weighing platform for applying a load to the load cell by means of a load transmission system pivotally mounted within the casing.
Capacitive load cells have been proposed for such weighing scales, provided in the form of a parallel plate capacitor whose capacitance varies inversely as a function of the distance between the capacitor plates. Consequently, as a load is applied to the weighing platform, the distance between the capacitor plates varies and the capacitance varies proportionally. A problem with such a system is to ensure uniformity of measurement, whereby the capacitance of the load cell changes by a uniform amount regardless of where on the weighing platform the load is applied.
One approach to this problem has been rigidly to connect the parallel plate capacitor to corresponding upper and lower portions of a rectangular spring whose side portions are rigidly coupled to the respective lower and upper sections of the casing. As a load is applied to the weighing platform, the two sections of the casing are displaced away from each other and thereby elastically deform the spring so as to move the two plates of the capacitor further apart. As the load is released, the spring returns to its original rectangular shape, thereby restoring the capacitance of the load cell to its original value.
Clearly, the resolution of such a measuring system depends on the proportional change in capacitance of the load cell for a given applied load. Consequently, for a capacitor of predetermined dimensions, the change in capacitance is a function of the magnitude of the applied load and the elastic modulus of the spring. In other words, the sensitivity of such an arrangement is a function of the elasticity of the spring. Consequently, an arrangement which is suitable for measuring an applied load of, say, 1 kgf, will generally not be suitable for measuring an applied load of 10 kgf unless the elastic modulus of the spring is increased by a factor of 10. If this is not done, then there exits the danger that the increased load will simply collapse the spring unrecoverably beyond its elastic limit.
However, in order to increase the rigidity of the spring as required (i.e. by increasing its elastic modulus), the dimensions of the spring must be increased accordingly in order that the increased load may be measured with the same sensitivity as the original load of 1 kgf. Such an arrangement, therefore, becomes cumbersome when measuring large loads since the dimensions of the spring as well as those of the weighing platform become unwieldy.